Grindstone and process of treating same



May 15, 1934.. w. H. MCGILL. 1,959,059

GRINDSTONE AND PROCESS OF TREATINr SAME 4 Filed Feb. 27, 1932 IATvTAOR/VEV.

Patented May 15, 1934 A GRmDs'roNE 1,959,059 AND PROCESS F TREATING SAMEwilliam n. McGill, Winchester, Mass. ,Application February 27, 1932,Serial No. 595,647 17 Claims. (Cl. 51-278) This invention relates togrindstones of the type made from natural stone. It is more especiallyconcerned with grindstones used in grinding wood in the process ofmaking pulp, paper, fiber board,

and the like,'although it will be evident that some features of theinvention can also be used -to advantage in grindstones employed for'other purposes. Y

The stones used in pulp grinding machines are of cylindrical form andare of relatively large dimensions. Usually they run from fifty-fourinches to sixty-seven inches indiameter and have a grinding face of atleast two feet in width, and usually more. A stone sixty-seven inches indiameter and fifty-four inches in width is not uncommon in thisindustry. Stones of this general character will be hereinafter referredto as fpulp grindstones.

Because of the large size -of these stones it is diicult to nd depositsof natural stone having the desired grinding characteristics from whichto cut grindstones of the desired dimensions. A particularly unfortunatecondition is the fact that those quarries in which the .larger stonescan be cut often yield a stone which does not have the necessarymechanical strength to stand up in use.

'I'he natural stone from which these grind- `stones are cut is a formofsandstone. When the grindstones are first cut from this material theyare soft and green and it is the usual custom to put themthrough aseasoning process which requires several months in thecase of largestones. During this process the stone dries out and its mechanicalstrength and hardness gradually increases, lreaching a maximum when theseasoning process has been completed. This seasoning can be doneartificially by baking thev stones in ovens two to three days at 300 to325 F. In the pulp grinding operation the stones are thoroughlysaturated with moisture due to the condition under which the woodgrinding operation is performed, and the water so applied attacksl orweakens the natural bond of the stone so that it loses a very largeproportion of its mechanical strength. This loss in strength variessomewhat with different kinds of stones, in fact, some varieties whichhave very satisfactory grinding characteristics'when dry cannot be usedin pulp grinding because of the, excessive loss of strength which they.undergo when wet. It may also be pointed out that. the stone in a pulpgrinder, in addition vto being thoroughly wet during practically itsentire life, must also operate at a relatively high degree of heat, thisheat being 55 crested by the friction of the stone -against the woodduring the grinding operation. While the temperature so produced willvary with different operating conditions, it will customarily runsomewhat under the boiling `point of water, say roughly from 160 F. to200 F., but it Will rise 60 above this value under severe conditions. Itis not unusual for partl of a stone to shell off o'r blister due to thehigh temperatures under which it operates, or to sudden changes intemperature.

-It is one of the objects of this invention to increase the wet strengthofgrindstones made of natural stone, and to this end it is proposed toimpregnate the stone with suitable waterproofing substances which willprotect the natural bond of the stone from the weakening action ofWater, or at least will materially reduce such action, and whichpreferably, also, will of itself give added mechanical strength andstability to the natural stone.

It has been proposed heretofore to increase the l strength and stabilityof artificial grindstones by soaking them in a solution of some binderand subsequently drying them out, or even baking them. A long series ofexperiments has demonstrated that methodsv of this character cannot beused successfully in grindstones made of natural stone. In fact, I havenot been able to learn that they have been successfully used onartificial stones. I have found, however, that the characteristics ofnatural stone suitable for grindstones are such that theycannot besatisfactorilyimpregnated with any binder of which I have been able tolearn, and which is suitable for the purposes here contemplated, by themethods suggested for the treatment of articial stones. This is largelydue to the fact that the natural stone is far more dense than is theartificial stone, its microscopic pores and cells being much smallerthan those in the latter stone. It therefore exerts a high filteringaction lon any impregnating medium. In addition, the 'composition ofnatural stonesis not entirely uniform, but there are variations in itsstructure which introduce serious problems in impregnating such stoneson a commercial basis.

' To devise a thoroughly satisfactory and reli- 100 able method ofimpregnating grindstones made of natural stone, therefore, forms afurther and Very important object of this invention.

Referring now to the drawing,

Figure 1 is a vertical, sectional view, partly in 105 elevation,illustrating a step in the process provided by this invention; and

Fig. 2 is a view similar to Fig. 1 but showing additional parts insection, and illustrating another step in said process.

According to the preferred method the grindstone 2 is thoroughly driedto drive out a high percentage of the moisture in it before theimpregnating operation is performed. For this purpose it may be baked ina suitable oven and at moderate temperature for a sufficient length oftime to heat the stone approximately throughout its mass. Naturally thelength of time required for this operation will-depend upon thecondition of the stone, its size, the temperature used,`and

y the like.

'Ihe impregnating operation is conveniently performed in a pressurecylinder or tank of some suitable design, such for example as that shownat 3 in Figs. 1 and 2. Before placing the stone in the tank, heads orplates 4 and 5, respectively, are clamped against the opposite ends ofthe stone, one of these plates having an integral shaft 6 which extendsthrough the bore ,7 of the stone 2-an'd is threaded to receive a nut forclamping the other plate in position. Gaskets 8-8 of rubscale at 10-10,Fig. 2.

When the stone has been prepared in this manner it is placed in the tank3, the lower plate 5 resting on bosses l2 located in the bottom of thetank. Preferably the bore 7 of the stone is vented, as for example, byproviding a hole 13 through the head 5 and gasket 8, equipping this headwith a suitable fitting 14, connecting a tube 15 to this fitting andleading it to a nipple 16 which projects through the wall of the tank.

When these operations have been performed the cover 3' is bolted inplace and the impregnating medium then is admitted to the tank throughthe pipe 1'7 and valve 18. It is desirable to have the impregnatingprocess follow the baking or drying operation promptly before the stonehas had an opportunity to absorb or occlude any substantial amount ofmoisture and preferably, also, while the stone is still warm so that themicroscopic pores and cells on the stone will be as empty as possible.If desired, a preliminary vacuum can be applied after the tank has beenclosed and before the impregnating liquid is admitted. After the stonehas been submerged in the liquid, air is pumped into the tank throughthe pipe 20 and exerts sufficient pressure on the liquid to force itinto the stone. Naturally the temperature at whichthe liquid ismaintained, the pressures used, and the treating period required willdepnd upon the conditions surrounding individual cases. In a typicalcase a pressure of twenty-five pounds is applied for the first half hourand this pressure`is stepped up ten pounds at the end of said period andto fifty pounds at the end of an hour, the latter pressure beingmaintained until the liquid begins to drip through the nipple 16, or,more desirably, until a gallon or two of liquid has run through thenipple. This means that practically the entire volume of the stone hasbeen thoroughly saturated with the impregnating liquid. A stonefifty-four inches in diameter and having a twenty-seven inch face will,in a. typical treatment, take about thirty gallons of liquid.

For most purposes it is desirable to leave the stone in a fairly porouscondition and to use as little of the impregnating medium as isnecessary to give the desired degree of strength to the stone.Consequently, I find it preferable in most cases to withdraw some of theimpregnating liquid from the stone after it has begun to run through thevent in sufficient quantity to indicate definitely that the properdegree of impregnation has been effected. This is accomplished bydraining the tank and then maintaining a sufficient air pressure, sayfor example fty pounds, in the tank until the desired amount ofimpregnating liquid has been forced out through the vent. For example,in a stone of the size just mentioned from six to eight gallonscustomarily are withdrawn through the vent after the tank has beenemptied.

This procedure has the important advantage of producing a much moreuniform impregnation of the stone, notwithstanding-natural variations inits structure which must be expected to exist, than could be obtainedwithout the use of this or some equivalent process. It improves theconcentration and distribution of treating material at the centralportion of the stone, approximately neutralizes the filtering action,partly empties the pores and cells, and avoids the tendency to produce ahigher concentration near the periphery of the stone. In effect thispart vof the process consists in forcing an excessive quantity oftreating liquid into the stone and then withdrawing the requiredproportion of that liquid by forcing it inwardly toward the bore 7. Thedesired distribution of the impregnating material also is facilitated bysizing the end faces of the stone with varnish or some other materialrelatively impervious to the impregnating medium and using the gaskets8-8 with the pressure plates 4 and 5 to hold them in place, thisarrangement serving to prevent any short circuiting of the liquid in thestone which otherwise would be likely to occur if these precautions werenot taken. Obviously it is not always necessary to size the entire endfaces of the stones, the portions of these faces adjacent to theperiphery of the stone being left unsized, if desired.

At the completion of the operations above described the stone usually istaken out of the tank and allowed to stand for several hours to drain.It is then heated or baked to drive off the solvent, in the event thatone has been used in the impregnating liquid.

The remaining steps of the process will depend upon the'nature of theimpregnating material employed. I have obtained the best results byusing a synthetic resin dissolved in a solvent as the impregnatingmedium. For example, a phenol-formaldehyde resin dissolved in denaturedalcohol preferably with a small percentage, say

.10%, of any of the well known plasticizers for such resins, has givenparticularly satisfactory results. A solution in the proportions of sixpounds of alcohol to one pound of resin will penetrate the stone quitereadily under the conditions above described even while carrying on theprocess at room temperatures. With such a material I prefer to bake thestone after it has been impregnated and drained, at a temperature ofaround, say, 200 F. until nearly all of the alcohol has been evaporated,then to step up the temperature to from 330 F. to 350 F. and to hold itat that point until the resin has been converted into its insoluble andinfusible form, it being understood that the resin in the impregnatingliquid is in one of its lower phases or stages and should be convertedinto a higher stageby heat after the impregnating operation has beencompleted. In a stonefifty-four inches by twenty-seven inches a bakingperiod of approximately two days at the temperatures above mentioned maybe necessary to carry the conversion of the resin to the desired point.With the particular impregnating medium just mentioned a hard gumvarnish or cumar resin varnish may be used satisfactorily for sizing theend faces of the stone.

While I have above described one method of procedure Which I have foundto produce satisfactory results, it will be evident that considerablevariation in the steps performed andin the materials used is permissiblewithin the spirit and scope of the invention. For example, aconsiderable variety of synthetic resins can be successively used inthis process. Also, when the normal operating temperatures are not assevere as those used in pulp grinding machines, other impregnatingsubstances such as high melting point waxes, particularly carnauba,candellila and montan may be used, those waxes having a sharp meltingpoint being preferable. Silicate of soda may also be used successfullyif the stones are baked sumciently after the treatment to make theresidue waterproof.

It will be evident that the degree of hardness or strengthimparted tothe stone by this process can be controlled by varying the amount ofimpregnating material left in the stone. This, in turn, can be regulatedby properly controlling the concentration of the solid constituents inthe impregnating liquid. Obviously too viscousa liquid cannot be usedsuccessfully, but the concentration can be varied very substantiallywhile still producing entirely satisfactory results and affording thedesired degree of control.

Such control can also be effected by suitably varying the amount of theimpregnating medium which is ejected from the stone after the pressuretreatment has been completed. Or, in other words, by governing thedegree to which the pores and cells in the stone will be filled withsaid medium. It should be borne in mind, however,

that it is desirable in any event to have the walls of the pores andcells thoroughly coated with the solid constituent of the impregnatingagent.

In some cases it may be desirable to have the impregnation confinedchiefiy to the wearing vol-.- umo of the stone and this can be done byreducing the time required in the pressure treatment and ejecting a partof the medium by'forcing air into the bore ofthe stone and out towardthe periphery. Much better results, however, are obtained by thepreferred method above described. It is also possible to reverse thedirection in which the impregnating material is introduced, but a bettercontrol of the process is eifected and more certain and uniform resultsare obtainable by following the preferred process.

This method enables me to make pulp grinding stones from grades ofnatural stone which have heretofore been rejected because of theirsoftness and lack of strength. In addition, the fact that the hardnessof the stones can be controlled and approximately predetermined by thisprocess is an extremely important practical advantage. When a syntheticresin or equivalent binder is used, any danger of the stone blisteringor shelling off is completely overcome since such binders are notaffected byany temperature at which the stone can be expected to operatein grinding pulp. Furthermore, since all of the binders above mentionedare waterproof they not only protect the natural bond of the stone butalso increase the strengthand hardness of the stone due to their ownbinding effect which is retained substantially unaltered even under verysevere operating conditions. It should also be noted that the preferredprocess above described drives the impregnating medium partly out of thepores and cells of the stone throughout the wearing area thereof so thatthese pores and cells are partly emptied while the' intercellularstructure remains coated with the binding material. desired porosity ofthe stone is maintained notwithstanding the impregnation.

Having thus described my invention, what I desire to claim as new is:

1. A pulp grindstone consisting of natural sandstone impregnatedthroughout the body thereof with a waterproof binder which retains itsbinding properties at operating temperatures.

2. A pulp grindstone consisting of natural sandstone impregnatedthroughout the body thereof with a waterproof binder which remains hardin the stone at operating temperatures up,

Consequently,- the proof lbinder tosubstantially increase the wet 5. Apulp grindstone consisting of.' natural Y sandstone impregnatedthroughout, the boch!l thereof with a waterproof synthetic resin.

6. That improvement in methods of increasingy the wet strength' of pulpgrindstones made of natural stone which consists in forcing a waterproofbinder in a liquid form under fluid pressure into the microscopic poresand cells of the grindstone throughout 'the greater part of the bodythereof while holding the grindstone substantially stationary,u andsubsequently withdrawing a substantial proportion of the material sointroduced to partly empty said microscopic pores 'and cells whileleaving the Walls of said vcells coated with the impregnating material.

7. That improvement in methods of impregnating grindstones ofapproximately cylindrical form and provided with a central bore whichvconsists in forcing the impregnating medium under pressure into thestone radially from the periphery thereof toward said bore, andcontinuing said operation until s aid medium exudes from the surface ofthe bore.

8. That improvement in methods of increasing the wet strength ofpulpgrindstones made of natural stone which consists in impregnating thedesired volume of the grindstone with a binder dissolved in a solvent,forcing the solution of said binder under uid pressure throughout saidvolume, while holding the grindstone substantially stationary, andsubsequently evaporating said solvent.

9. That improvement in methods of increasing binder under fluid pressurethroughout said volume, while holding the grindstone substantiallystationary withdrawing from said stone a substantial proportion of theimpregnating medium so introduced, and subsequently evaporating saidsolvent.

10. That improvement in methods of impregnating grindstones ofapproximately cylindrical form and provided with a central bore whichconsists in submerging said stone in a body of liquid impregnatingmedium, venting the bore of the stone so submerged, forcing said mediumunder pressure through said stone radially thereof until it exudes fromthe surface of said bore, and preventing any substantial lateralpenetration of said medium into said stone in regions adjacent to saidbore.

11. That improvement in methods of impregnating grindstones ofapproximately cylindrical form and provided with a. central bore whichconsists in submerging said stone in a body of liquid impregnatingmedium, Venting the bore of the stone so submerged, forcing said mediumunder pressure through said stone radially thereof until said medium haspenetrated through approximately the entire volume of said stone,preventing any substantial lateral penetration of said medium into saidstone in regions adjacent .to said bore, subsequently withdrawing saidbody of impregnating medium, and applying a. sufficient gaseous pressureto the impregnated stone to force a substantial proportion of theimpreg'nating medium still held in the stone out through said bore.

12. That improvement in methods of increasing the wet strength of pulpgrindstones made of natural sandstone, and provided with v'a bore, whichconsists in confining the stone in a closed container, and forcing abinder in a liquid form and under pressure, solely from the periphery ofthe stone toward said bore, into the microscopic pores and cellsthroughout the greater part of the wearing volume of said stone while itis so confined.

13. That improvement in methods of increasing the wet strength of pulpgrindstones made of natural sandstone, and provided with a bore,

which consists in withdrawing the moisture and gases from themicroscopic pores and cells(y throughout the greater part of the body ofthe grindstone to at least partially empty said pores and cells;submerging the stone so treated, and

, while the pores arid cells thereof are partially stone throughout thegreater part of the wearing volume of the stone. f

14. That improvement in methods of increasing the wet strength of pulpgrindstones made of natural sandstone, and provided with a bore, whichconsists in withdrawing the moisture and gases from the microscopicpores and cells throughout the greater part of the body of thevgrindstone to at least partially empty said pores and cells; submergingthe stone soA treated, and while the pores and cells thereof arepartially emptied, in a bath of binding material in a liquid form; andapplying sufficient pressure to said bath to force it, solely from theperiphery of the stone toward said bore, into said pores and cells ofthe stone throughout the greater part of the wearing volume of thestone, said binder being of such a nature as to cure under the action ofheat, removing the stone from said bath, and subsequently baking thestone so impregnated to cure the binder.

15. 'I'hat improvement in methods of increasing the wet strength of pulpgrindstones made of natural sandstone, and provided with a bore, whichconsists in confining the tone in a closed container, forcing asynthetic resin dissolved in a solvent under pressure, solely from theperiphery of the stone toward said bore, into the microscopic pores andcells of `said stone throughout the greater part of the wearing volumethereoff while the stone is so confined, said resin being in one of'itslower stages, and subsequently heating said stone sufiiciently toevaporate said solvent and to convert said resin into a higher stage.

16. That improvement in methods'of impregnating grindstones ofapproximately cylindrical form and provided with a central bore, whichconsists in forcing the impregnating medium under pressure into thestone in a generally radial direction, inwardly thereof, and closing thestone over annular areas at the ends of the stone, surrounding theopposite ends, respectively, of said bore, against any substantialpenetration of said medium laterally into the stone.

.17. That improvement in methods of impregnating grin'dstones ofapproximately cylindrical form and provided' with a central bore which.consists in forcing the impregnating medium under pressure into thestone in a generally radial inward direction, continuing said operationuntil said medium has been forced through the entire radial dimension ofthe stone, maintaining a substantially greater pressure on said mediumat the peripheral surface thereof than at the surface of said bore, andclosing annular areas surrounding sad bore and located at-the end'surfaces of the stone against any substantial penetration of saidmedium laterally into the stone.

' WILLIAM H. McGILL.

